The transmission of signals through optical fibers over long distances has been made possible largely because the losses in modern glass fibers are small. The lower losses in an optical fiber as compared to a metallic cable reduce the numbers of repeaters required in a transmission length, which improves the reliability and reduces the complexity of the link. Low-loss optical fibers with high bandwidths are revolutionizing long-haul communications for both commercial and military applications.
Losses in optical fibers are determined by spectral-attenuation, which is intrinsic to the fiber design, and bending losses which result from externally applied stresses. Spectral attenuation is determined by absorption and scattering, which have been minimized over the years through improvements in the glass composition and manufacturing processes. Bending losses are the result of optical power leaking out of the optical fiber where sharp bends occur in the fiber. A consequence of these bending losses is a reduction of the allowed transmission's distance.
To prevent severe bends, optical fibers have been inserted into cables with increased reinforcement and stiffness. However, a limitation of these reinforced cables is that their design dilutes the flexibility and lightweight advantages inherent in optical fibers when compared to the metallic cables. Structural modifications of the fiber itself have been attempted in an attempt to minimize bending loss, yet, for the most part, these attempts have resulted in an undesirable increase in spectral attenuation.
Thus, a continuing need exists in the state of the art for a pure-silica-core dual-mode fiber which exhibits both low bending losses and low spectral attenuation to produce a unique capability for long-haul fiber optic transmission in severe bending environments such as those encountered in undersea cables and missile tethers.